Scanning system for cathode-ray tubes



p 1952 R. R. THALNER 2,612,622

SCANNING SYSTEM FOR CATHODE-RAY TUBES Filed Dec. 14, 1950 l fiyi' l0 may FREQ. AMPL. VIDEO DETECTOR u 1 1050 AMPLIFIER.

SYNC/IRO/V/Zl/VG VERTICAL .S'IGWAL SCANNING .S'FPARATOR srsn-w I3 $0.1m; OF E HORIZONML' I 5C'AAW/A/6 s/a/v/rw I 26 Z i 27 Egtg 7 i 33 /7- W L INVENTOR 'ROBERT R. THALNER Patented Sept. 30, 1952 scsNN No SYSTEM F TUBE Rebel-tit. Thalner, Bufialo, N. Y., assignor to,

oa cA'rHonE- Rar Sylvania Electric Products Inc., a. corporation of Massachusetts Application December 14, 1950, Serial No: 200,712

The present invention relates to scanning sys= tems for cathoderay tubes and, particularly,to magnetic scanning systems such as employed in the horizontal scanning system of a television receiver. i

The horizontal scanning system of a conven tional television receiver ordinarily supplies currents of saw-tooth waveform to a horizontal de-' fiection winding of a scanning yoke positioned about the neck of a cathode-ray tube type of image-reproducing device by which to effect horizontal deflection of the electron beam of the tube. The horizontal scanning winding is ordinarily comprised of two, sections, and the electrical connections of these sections and their mechanical positioning causes one of the winding sections to have a larger value of inherent capacitance at its terminals than does the other section. The frequency of the saw-tooth scanning currents is sui ficiently high that this unbalanced capacitance tends to cause an undesirable form of uncontrolled or free oscillation during the scanning retrace interval which may persist for a short time at the beginning of the trace interval and thereby undesirably aifect the scanning action of the system.

It has heretofore been proposed that the effect of the unbalanced capacitances across the hori zontal scanning winding sections of the scanning yoke be reduced by connecting the two sections in parallel, but this requires that the windings be formed of an appreciably larger number of turns of smaller sized wire as compared to a scanning 5 Claims. (crew-27)" winding in which the two sections are series connected. There is also the disadvantage that the large voltage, of the order of several thousand.

Volts, generated across the scanning winding dur- It is an object of the present invention to provide a new and improved magnetic scanning system in which the undesirable effect of any unbalanced inherent capacitances of the horizontal scanning winding sections is substantially reduced.

It is a further object of the invention to provide a novel magnetic scanning system of simple and inexpensive construction and one in which innerent unequal capacitances across the horizontal i 2 scanning winding sections have little or no efiect upon the operation of the system.

For a better understanding of the present invention, together with other and furthe-robjects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scopewill be pointed out in the appended claims.

Referring now to the drawings, Fig. l is a circult diagram, partly schematic, representing a complete television receiver embodying the present invention in a particular form; Fig. 2 is a circuit diagram representing a portion of a scanning system embodying the inventioninv a modi fied form; and Fig. 3 also is a portion of a scanning system embodying the invention in an additionally modified form.

Referring now more particularly to Fig. l, the television receiver includes a unit It having an input circuit coupled to an antenna H and an output circuit coupled to the intensity control electrodes of a cathode-ray tube type of image reproducer I2. The unit It] is conventional and includes a high frequency amplifier, a video detector, and a video amplifier. The picture signal developed in the output circuit of the video detector of unit 10 is applied to a synchronizingsignal separator [3 where the synchronizing signal components of the picture signal are separated from each other and from the video components thereof, the vertical synchronizing components being applied to control the synchronized operation of a vertical scanning system it. The

output circuit of the latter unit is coupled to a.

vertical deflecting winding l5 of a scanning yoke positioned in'a conventional manner about the neck of the image reproducer l2. The horizontal synchronizing signal components, separated from the television signal by the unit l3, are applied to a source of horizontal scanning signals E6 to control the synchronized operation thereof. The unit I6 is conventional, and the outputcirc'uit of this unit is coupled to a'scanning amplifier ii, the detailed arrangement and operation of which will be described hereinafter. The output circuit of unit it is coupled to a horizontal scanning winding [8 of the deflection yoke.

With the exception of the scanning amplifier ii, the construction and operation of the television receiver thus fardescribed is conventional in that a television signal received by the antenna H is applied to the unit II] where it is amplified, detected, and again amplified after detection before being used to control the intensity of the cathode-ray beam of the image-reproducing tube 12. The vertical and horizontal synchronizing components of the received signal are separated by the separator I3 and are applied to the respective vertical scanning system it and source of horizontal scanning signals It to; control the synchronized operation thereof. The scanning signals applied from the output circuit of the unit 14 to the vertical deflection winding and the scanning signal applied to the horizontal scanning winding I8 through the amplifier [1 from the source H5 cause the cathode-ray beam of the tube l2 to trace a raster of horizontal lines I to reproduce the television image.

Referring now more particularly to the portion of the receiver embodying the present invention, the unit 11 includes an amplifier tube 19 having input electrodes coupled to the output circuit of the unit 16 and having output electrodes coupled through acondenser 20 to the horizontal scanning winding [8. The amplifier tube I9 is energized from a source of uni-directional current 2| through an adjustable horizontal size-control resistor 22 and an iron-core choke coil 23. A conventional diode type of damper tube D is coupled across the output circuit of the amplifier 19 for well known reasons.

The horizontal scanning winding i8 is centertapped, and undesirably has unequal'inherent distributedicapacitances across the two halves thereof. To reduce the undesirable effect of these unequal distributed capacitances on the scanning action effected by the scanning system, the center tap of the scanning winding [8 isaccupled through a conductor 24 and a resistor 25 to the .center tap 26 of a. choke coil 21 which is inductively coupled to thechoke coil 23. The choke coils 23 and 21 have: the" same number of turns of wire and are tightly coupled magnetically. Centering current for the deflecting winding I8 is provided by a voltage divider 28 the resistive element of which is coupled through a resistor 29 across a portion of the uni-directional current source 2| and the movable contact .36,

of which is connected to one end of the choke coil 21. .A by-pass condenser 3| is connected across the resistive element of the voltage divider 28 to by-pass'around'th'e latter any alter"- nating current components of scan frequency.

Considering now the operation of the scanning system just described, the source I6 and amplifier l9 comprise a sourceof scanning signals for effecting energization of the scanning winding IS with currents of linear saw-tooth scanso-called high-impedance type in which the diode device D is continuously conductive and current is supplied-to the scanning winding 18 substantially only during. the latter half. of the relatively long trace interval of the saw-tooth scanning signal. During the initial portion of the trace interval, the energy stored in the scanning winding I'B discharges at a constantly decreasing rate through the diode device D. The latter also serves to damp. out any undesired transient oscillations which may occur at the end of the relatively short retrace interval of the saw-tooth scanning signal.

Where the scanning winding l8 has two sections or halves connected in'series with aiding magnetic fields as shown, it is well known that the inherent distributed capacitance across one half of the winding is different. from that across the other. This inequality of the inherent capacitances tends to. create rather severe transient oscillations during and immediately following the retrace interval of each scanning cycle, and such oscillations undesirably create a pattern of verticalspaced bars atthe lefthand side of-the reproduced image. It is the purpose of the present invention to avoid this effect 01- ning waveform. This scanning. system is of the Y the scanning winding l8.

the unequal distributed capacitances appearing .across the two halves of the'scanning winding 18. This is accomplished in accordance with the present invention by connecting the center tap of the winding I8 to the center tap 26 of the choke coil v21 so that the voltages from the center tap to each terminal of the winding 18 must be identical at every instant with the voltages ing' the center tap of the winding 13 from the center-tapped choke coil 21, the unequal inherent capacitances across the two halves of the scanning winding l8 are no longer able to modify or afiect the voltages applied to the two halves of the scanning winding. While the center tap of the scanning winding [8 could be directly connected by the conductor 24 to the center-tap 26 .of the choke coil 21, insofar as the operation 'lastdescribed is concerned, the resistor 25 prevents the connection last mentioned from causing the centering current supplied from the volt.- age divider 28 from flowing through only one half of the scanning winding 18. That is, the resistor 25 is selected to have a resistanc of approximately 1,000 ohms, or roughly ten times the combined ohmic resistance of the choke coil 21 and scanning winding 18, so that the centering current flows through both halves of the scanning winding I8 in series and little of the centering current'fiows through the resistor 25. By the same token, the resistor 25 has a sufficiently small value with relation to the impedance of the scanning winding I 8 at the frequency of the scanning signal as to be negligible insofar as the scanning operation. is concerned and particularly insofar as the reduction of the undesirable en'ect of the'unequal distributed capacitances on the two halves of the scanning winding I8 is concerned.

Fig. 2 is a circuit diagram representing a portion of the scanning system of Fig. 1, elements of Fig. 2 corresponding to similar elements of Fig. 1 being designated by similar reference numerals, except that the scanning system of Fig. 2 utilizes a conventional horizontal output transformer 33 having a primary winding 34 included in the output circuit of the amplifier I9 and having a secondary winding 35 coupled to the scanning winding I8. Further by Way of modification, the center tap 36 of the secondary winding 35 is coupled through a condenser 31 to the center tap of the winding I 8. Centering current is supplied from the voltage divider 28 which has a fixed center tap 32 coupled to one end of the scanning winding l8 and has its movable contact 30 coupled to on end of the secondary winding 35. The operation or thi modified form of the invention is essentially similar to that described in connection with Fig. l, the condenser 34 being used tightly to couple the center tap of the scanning winding 18 to the center tap 36 of the secondary winding 35 of the horizontal output transformer 33 for currents of scanning frequency. This reduces the undesirable effect on the operation of the scanning system of the unequal inherent capacitances appearing across the two halves of At the same time, the condenser 31 insures that centering current supplied from the potential divider 28 flows through the two halves of the winding 18 in series. The use of a coupling condenser 31 requires that the secondary transformer winding 35 and the scanning winding 13 be quite carefully center-tapped electrically, as is also the case where the center taps of the windings l8 and 35 are directly connected by a conductor. With either the use of the coupling condenser 3'! or a direct connection between the center taps of the windings l8 and 35. an electrical unbalance of the center taps of approximately 10% or less can be tolerated. Larger percentages of unbalance than this are satisfactory when the resistor of the Fig. 1 arrangement is used.

Fig. 3 is a circuit diagram of a portion of the scanning system as in Fig. 2 except that the condenser 31 and the resistor 25 are both used between the center tap 36 of the secondary winding and the center tap of the scanning winding 10. The condenser 31 isolates the center taps of the windings l3 and 35 insofar as centering current is concerned, and this permits the resistor 25 to have a smaller value than in an arrangement of the Fig. 1 type thus more tightly to couple the center taps of the windings l8 and 35 insofar as currents of scanning frequency are concerned. The operation of the Fig. 3 arrangement is otherwise essentially similar to that hereinbefore described.

While there have been described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the invention. Consequently, the appended claims should be interpreted broadly, as may be consistent with the spirit and scope of the invention.

What I claim is: i

1. A magnetic scanning system for a cathoderay tube comprising, a center-tapped scanning winding for said tube to deflect the cathode-ray beam thereof in one direction, said winding having similar physical winding structures on each side of said center tap but undesirably having unequal inherent distributed capacitances across the two halves thereof, a source of scanning signals for effecting energization of said winding with currents of scanning waveform, and an impedance coupling said source and said winding and including a winding having a center tap coupled through a resistor to the center tap of said scanning winding to reduce the undesirable effect of said unequal distributed capacitances on the scanning action effected by said system.

2. A magnetic scanning system for a cathoderay tube comprising, a center-tapped scanning winding for said tube to deflect the cathode-ray beam thereof in one direction, said winding having similar physical winding structures on each side of said center tap but undesirably having unequal inherent distributed capacitances across the two halves thereof, a source of scanning signals for efiecting energization of said winding with currents of scanning waveform, and an impedance coupling said source and said winding and including a winding coupled through a coupling condenser and resistor in series to the center tap of said seaming winding to reduce the undesirable eiiect of said unequal distributed capacitances on the scanning action effected by said system.

3. A magnetic scanning system for a cathoderay tube comprising, a center-tapped scanning winding for said tube to deflect the cathode-ray beam thereof in one direction, said winding having similar physical winding structures on each side of said center tap but undesirably having unequal inherent distributed capacitances across the two halves thereof, a source of scanning signals for effecting energization of said winding with'currents of scanning waveform, a transformer having a primary winding in the output circuit of said source and a secondary winding coupled across said scanning winding, said secondary winding having a center tap coupled through a resistor to the center tap of said scanning winding to reduce the undesirable efiect of said unequal distributed capacitances on the scanning action effected by said system.

4. A magnetic scanning system for a cathoderay tube comprising, a center-tapped scanning winding for said tube to deflect the cathode-ray beam thereof in one direction, said winding having similar physical winding structures on each side of said center tap but undesirably having unequal inherent distributed capacitances across the two halves thereof, a source of scanning signals for effecting energization of said winding with currents of scanning waveform, a transformer coupled to said source and including a winding section coupled across said scanning winding, said winding section having a center tap coupled through a resistor to the center tap of said scanning winding to reduce the undesirable efiect of said unequal distributed capacitances on the scanning action effected by said system.

5. A magnetic scanning system for a cathoderay tube comprising, a center-tapped scanning winding for said tube to deflect the cathode-ray beam thereof in one direction, said winding having similar physical winding structures on each side of said center tap but undesirably having unequal inherent distributed capacitances across the two halves thereof, a source of scanning signals coupled through a condenser to said scanning winding for effecting energization of said winding with currents of scanning waveform, an inductor included in the output system of said source for supplying uni-directional energy thereto, and an inductor inductively coupled to said last-mentioned inductor and efiectively connected in shunt to said scanning winding tosupply a uni-directional centering current thereto, said second inductor having a center tap coupled through a resistor to the center tap of said scanning winding to reduce the undesirable efiect of said unequal distributed capacitances on the scanning action efiected by said system.

ROBERT R. TI-IALNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Morgan July 3, 1951 

